1. Field of the Invention
The present invention relates to magnesium-based catalyst precursors, methods of preparing the catalyst precursors, and methods of using the catalyst precursors in catalyst systems to produce polyolefins, particularly polyethylene, more particularly medium density polyethylene (MDPE) and linear low density polyethylene (LLDPE).
2. Description of the Related Art
Polyethylene polymers are well known and useful in many applications. In particular, linear polyethylene polymers possess properties that distinguish them from other polyethylene polymers, such as branched ethylene homopolymers commonly referred to as LDPE (low density polyethylene). In the polyethylene industry, linear polyethylene polymers, from linear low density polyethylene (LLDPE) to medium or higher density polyethylene (MDPE or HDPE) are used in a wide variety of applications including film forming, injection molding, rotomolding, and wire and cable fabrication. As compared with counterpart LDPE resins, such linear polyethylene polymers typically exhibit enhanced high dart impact, enhanced Elmendorf tear, enhanced tensile strength and enhanced elongation in both the machine direction (MD) and the transverse direction (TD).
Ziegler-Natta type catalyst systems for the polymerization of ethylene and other olefins are well known in the art, as illustrated by U.S. Pat. No. 3,113,115. Ziegler-Natta type catalysts are particularly useful for producing polyethylene polymers in both a slurry process and a gas phase process. Advanced Ziegler-Natta catalysts based on supported titanium systems have received industrial interest for producing high performance polyethylene resins. Examples of such catalyst systems are described in U.S. Pat. Nos. 4,105,585, 5,047,468, 5,091,363, 5,192,731, 5,260,245, 5,336,652, 5,561,091, and 5,633,419 and in European Patent Applications EP-0,529,977A1, EP-0,336,545B1, and EP-0,703,246A1 all of which are herein incorporated by reference.
As an example of such Ziegler-Natta type catalyst systems, catalysts prepared in-situ by reacting magnesium metal with at least one halogenated hydrocarbon and at least one tetravalent titanium compound have been described. Reacting magnesium metal powder with butyl chloride in a non-polar solvent in the presence of TiCl4/Ti(OR)4 to form a catalyst for gas phase ethylene co-polymerization has been disclosed. Advantages of this synthesis method for preparing Ziegler-Natta catalyst are formation of homogeneous active sites and simplified preparation procedure. However, these catalysts can show broad particle size distribution, poor morphology, poor operability for producing lower density resins, and inferior comonomer incorporation. The LLDPE resins obtained using such catalysts do not have the narrow molecular weight distribution and compositional distribution that are desirable for high performance resins. Moreover, these catalyst compositions cannot produce LLDPE with a density of less than 0.917 at economically favorable production rates because of poor powder flowability. In particular, in the gas phase process this catalyst composition produces polyethylene polymer with higher electric static and higher extractable fraction, which results in resin stickiness, chunk formation, and reactor fouling at economically favorable production rates.
Other supported titanium catalyst systems for LLDPE are obtained by dissolving MgCl2 with [TiCl3(AlCl3)1/3] in tetrahydrofuran (THF) to make a solution containing MgCl2 and titanium halide that is subsequently immobilized on a silica support. A process wherein MgCl2 is dissolved in an electron donating solvent and reacted with alkylaluminum compounds to solidify magnesium halide with aluminum alkoxy compounds has also been disclosed. The solid is then contacted with titanium halide to give a solid catalyst with effective co-polymerization ability. However, the preparation of such catalyst systems often requires complicated processing steps, and the LLDPE products obtained using these catalyst systems do not possess the narrow molecular weight distribution and the compositional distribution required for high performance resins. This inadequate molecular weight and compositional distribution presumably exists because of broadly inhomogeneous active sites in such catalyst systems.
Also, a catalyst system has been disclosed in which dialkylmagnesium and silane compounds are reacted with an —OH group on a silica support which is then contacted with a transition metal halide to form a relatively homogeneous active site. This silica supported catalyst system exhibits more homogeneous ethylene polymerization or co-polymerization capability than the previously discussed magnesium-based supported titanium halide catalyst systems as measured by resin MWD and compositional distribution. However, such catalyst systems require extra processing steps because the silica support must be treated, either chemically or thermally, to remove bound water and excess —OH groups prior to the formation of the catalyst.
Additionally, catalyst systems have been disclosed in which dialkylmagnesium compounds are impregnated into a silica support containing —OH groups to form a first reaction product. The first reaction product is then halogenated with HCl to convert the organomagnesium derived compound to MgCl2 thereby forming a second reaction product. The second reaction product is then treated with a transition metal halide such as TiCl4, a particular type of electron donor, and at least one Group 2 or 13 organometallic compound, such as diethylaluminum chloride. The multi-step process of this catalyst preparation is complicated and is a difficult process to use to provide controlled, stable catalyst quality.
To summarize the prior art, the preparation of Ziegler-Natta catalysts for the catalytic control of molecular weight or composition branching distribution has heretofore required complicated control of the active site formation process and careful tuning of the catalyst precipitation process to ensure formation of uniform catalyst active sites and consistent catalyst properties. Deteriorated catalyst properties are often present in the absence of control over the precipitation process, especially in multi-step processes.
Therefore, there is a need for developing catalyst systems that can be used to produce LLDPE with improved physical and chemical properties useful in a wide variety of products and applications. It would be advantageous to provide catalyst systems with the following characteristics: (1) enhanced catalyst activity and catalyst productivity; (2) narrower molecular weight distribution for polymer resins produced with the catalyst systems; (3) enhanced capability of such catalyst systems to co-polymerize ethylene and alpha-olefins; (4) reduced lower molecular weight component in polymer resin produced with the catalyst system; (5) enhanced short chain branching distribution (SCBD) or branching homogeneity in polymer resin produced with the catalyst system; (6) enhanced hydrogen response of the resin molecular weight; (7) reduced electron static during gas phase polymerization; (8) enhanced morphology and flow-ability; and (9) enhanced operation efficiency to produce LLDPE resins of lower density without resin stickiness, chunk formation, and reactor fouling in the fluidized bed gas-phase process, especially at high production rates.